Application of the Bjerrum Association Model to Electrolyte Solutions. IV. Apparent Molar Heat Capacities and Compressibilities in Water and Aprotic Solvents

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Abstract

The Bjerrum association model, which has already been applied successfully to volumes and enthalpies of dilution of electrolyte solutions, has now been extended to apparent molar heat capacities and compressibilities of these systems. The proposed method of calculation, which takes into account the relaxation effect observed in second derivatives of the excess Gibbs free energy, can be used to extrapolate to infinite dilution the experimental data for systems showing a wide range of association constants in acetonitrile, propylene carbonate, and water. The concentration dependence of the thermodynamic properties can be reproduced quantitatively by the addition of one or two virial coefficients. Literature data for CP,2,ψ and KS,2,ψ of electrolytes in aprotic solvents were refitted with this equation. For dissociated or slightly associated systems (KA < 10), the standard infinite dilution quantities (Y2o) are in excellent agreement with literature values. For systems with high KA, Y2o obtained by the model are systematically lower than those reported in the literature. This is not surprising, since the traditional method of extrapolation using the Debye–Hückel limiting law or the Pitzer equation does not take association into account. A computer software that performs the calculations for the application of the Bjerrum model to thermodynamic properties has been designed and is presented in the appendix.

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